the only reason "service brakes soon" comes on alone (no abs light) is if the fluid is low (or it thinks it is low i.e. faulty fluid level sensor) or the prop valve is stuck.
in either case you won't have a code, your only indication will be with a advanced scanner such as a tech2 that can read the brake fluid level status
as a test try unplugging the brake fluid level sensor, if the message goes away then it's likely the sensor unless you had brake work done lately and the prop valve got stuck in the process, in this case you can try bleeding the brakes slowly especially the rear. if there was no brake work done recently and the message is gone with the sensor unplugged then I would just replace the fluid sensor (easy fix)
another thing to try if the prop valve is suspect (again only if work was done recently) is to drive and do several emergency stops, hit the brakes real hard, sometimes this unjams the prop valve.
